Thin film transistors (TFTs) are widely used as switching elements in electronics, for example, in active-matrix liquid-crystal displays, smart cards, and a variety of other electronic devices and components thereof. The thin film transistor (TFT) is an example of a field effect transistor (FET). The best-known example of an FET is the MOSFET (Metal-Oxide-Semiconductor-FET), today's conventional switching element for high-speed applications. Presently, most thin film devices are made using amorphous silicon as the semiconductor. Amorphous silicon is a less expensive alternative to crystalline silicon. This fact is especially important for reducing the cost of transistors in large-area applications. Application of amorphous silicon is limited to relatively low speed devices, however, since its maximum mobility (0.5-1.0 cm2/V sec) is about a thousand times smaller than that of crystalline silicon.
Although amorphous silicon is less expensive than highly crystalline silicon for use in TFTs, amorphous silicon still has its drawbacks. The deposition of amorphous silicon, during the manufacture of transistors, requires relatively costly processes, such as plasma enhanced chemical vapor deposition and high temperatures (about 360° C.) to achieve the electrical characteristics sufficient for display applications. Such high processing temperatures disallow the use of substrates for deposition that are made of certain plastics that might otherwise be desirable for use in applications such as flexible displays.
In the past decade, organic materials have received attention as a potential alternative to inorganic materials such as amorphous silicon for use in semiconductor channels of TFTs. Organic semiconductor materials are simpler to process, especially those that are soluble in organic solvents and, therefore, capable of being applied to large areas by far less expensive processes, such as spin coating, dip coating and microcontact printing. Furthermore organic materials may be deposited at lower temperatures, opening up a wider range of substrate materials, including plastics, for flexible electronic devices. Accordingly, thin film transistors made of organic materials can be viewed as a potential key technology for plastic circuitry in display drivers, portable computers, pagers, memory elements in transaction cards, and identification tags, where ease of fabrication, mechanical flexibility, and/or moderate operating temperatures are important considerations.
Organic materials for use as potential semiconductor channels in TFTs are disclosed, for example, in U.S. Pat. No. 5,347,144 to Garnier et al., entitled “Thin-Layer Field-Effect Transistors with MIS Structure Whose Insulator and Semiconductors Are Made of Organic Materials.”
Considerable efforts have been made to discover new organic semiconductor materials that can be used in TFTs to provide the switching and/or logic elements in electronic components, many of which require significant mobilities, well above 0.01 cm2/Vs, and current on/off ratios (hereinafter referred to as “on/off ratios”) greater than 1000. Organic TFTs having such properties are potentially capable of use for electronic applications such as pixel drivers for displays and identification tags. Most of the compounds exhibiting these desirable properties are “p-type” or “p-channel,” however, meaning that negative gate voltages, relative to the source voltage, are applied to induce positive charges (holes) in the channel region of the device.
As an alternative to p-type organic semiconductor materials, n-type organic semiconductor materials can be used in TFTs as an alternative to p-type organic semiconductor materials, where the terminology “n-type” or “n-channel” indicates that positive gate voltages, relative to the source voltage, are applied to induce negative charges in the channel region of the device.
Moreover, one important type of TFT circuit, known as a complementary circuit, requires an n-type semiconductor material in addition to a p-type semiconductor material. See Dodabalapur et al. in “Complementary circuits with organic transistors” Appl. Phys. Lett. 1996, 69, 4227. In particular, the fabrication of complementary circuits requires at least one p-channel TFT and at least one n-channel TFT. Simple components such as inverters have been realized using complementary circuit architecture. Advantages of complementary circuits, relative to ordinary TFT circuits, include lower power dissipation, longer lifetime, and better tolerance of noise. In such complementary circuits, it is often desirable to have the mobility and the on/off ratio of an n-channel device similar in magnitude to the mobility and the on/off ratio of a p-channel device. Hybrid complementary circuits using an organic p-type semiconductor and an inorganic n-type semiconductor are known, as described in Dodabalapur et al. (Appl. Phys. Lett. 1996, 68, 2264.), but for ease of fabrication, an organic n-channel semiconductor material would be desired in such circuits.
Only a limited number of organic materials have been developed for use as a semiconductor n-channel in TFTs. One such material, buckminsterfullerene C60, exhibits a mobility of 0.08 cm2/Vs but is considered unstable in air. See R. C. Haddon, A. S. Perel, R. C. Morris, T. T. M. Palstra, A. F. Hebard and R. M. Fleming, “C60 Thin Film Transistors” Appl. Phys. Let. 1995, 67, 121. Perfluorinated copper phthalocyanine has a mobility of 0.03 cm2/Vs, and is generally stable to air operation, but substrates must be heated to temperatures above 100° C. in order to maximize the mobility in this material. See “New Air-Stable n-Channel Organic Thin Film Transistors” Z. Bao, A. J. Lovinger, and J. Brown J. Am. Chem, Soc. 1998, 120, 207. Other n-channel semiconductors, including some based on a naphthalene framework, have also been reported, but with lower mobilities. See Laquindanum et al., “n-Channel Organic Transistor Materials Based on Naphthalene Frameworks,” J. Am. Chem, Soc. 1996, 118, 11331. One such naphthalene-based n-channel semiconductor material, tetracyanonaphthoquino-dimethane (TCNNQD), is capable of operation in air, but the material has displayed a low on/off ratio and is also difficult to prepare and purify.
Aromatic tetracarboxylic diimides, based on a naphthalene aromatic framework, have been demonstrated to provide, as an n-type semiconductor, n-channel mobilities up to 0.16 cm2/Vs using top-contact configured devices where the source and drain electrodes are on top of the semiconductor. Comparable results could be obtained with bottom contact devices, that is, where the source and drain electrodes are underneath the semiconductor, but a thiol underlayer needed to be applied between the electrodes, which had to be gold, and the semiconductor. See Katz et al. “Naphthalenetetracarboxylic Diimide-Based n-Channel Transistor Semiconductors: Structural Variation and Thiol-Enhanced Gold Contacts” J. Am. Chem. Soc. 2000 122, 7787; “A Soluble and Air-stable Organic Semiconductor with High Electron Mobility” Nature 2000 404, 478; Katz et al., European Patent Application EP1041653 or U.S. Pat. No. 6,387,727. In the absence of the thiol underlayer, the mobility of the compounds of Katz et al. was found to be orders of magnitude lower in bottom-contact devices. U.S. Pat. No. 6,387,727 B1 to Katz et al. discloses fused-ring tetracarboxylic diimide compounds, one example of which is N,N′-bis(4-trifluoromethyl benzyl)naphthalene-1,4,5,8,-tetracarboxylic acid diimide. Such compounds are easier to reduce. The highest mobilities reported in U.S. Pat. No. 6,387,727 B1 to Katz et al. was between 0.1 and 0.2 cm2/Vs, for N,N′-dioctyl naphthalene-1,4,5,8-tetracarboxylic acid diimide.
Relatively high mobilities have been measured in films of naphthalene tetracarboxylic diimides having linear alkyl side chains using pulse-radiolysis time-resolved microwave conductivity measurements. See Struijk et al. “Liquid Crystalline Perylene Diimides: Architecture and Charge Carrier Mobilities” J. Am. Chem. Soc. Vol. 2000, 122, 11057.
Ser. No. 11/263,111 (not yet published) to Shukla et al. discloses a thin film of organic semiconductor material that comprises an N,N′-diaryl-substituted naphthalene-based tetracarboxylic diimide compound having a substituted or unsubstituted carbocyclic aromatic ring system directly attached to each imide nitrogen atom, wherein the substituents on at least one or both of the aromatic ring systems comprises at least one electron donating organic group.
U.S. Patent Publication 2006-0237712 A1 to Shukla et al. discloses a thin film of organic semiconductor material that comprises an N,N′-di(arylalkyl)-substituted naphthalene-based tetracarboxylic diimide compound having a substituted or unsubstituted carbocyclic aromatic ring system attached to each imide nitrogen atom through a divalent hydrocarbon group, wherein any optional substituents on the aryl rings comprises at least one electron donating organic group.
Ser. No. 11/285,238 to Shukla et al. discloses a thin film of organic semiconductor material that comprises an N,N′-dicycloalkyl-substituted naphthalene-1,4,5,8-bis(dicarboximide) compound having a substituted or unsubstituted aliphatic carbocyclic (alicyclic) ring system attached to each imide nitrogen atom in which an optional substituent or substituents on each ring comprises at least one electron donating organic group.
The above-mentioned organic thin films by Shukla et al. are capable of exhibiting, in the film form, the highest known field-effect electron mobility compared to reported values, up to 5.0 cm2/Vs, for known n-type compounds.
US Patent Pub. No. 2002/0164835 A1 to Dimitrakopoulos et al. discloses improved n-channel semiconductor films made of perylene tetracarboxylic acid diimide compounds, as compared to naphthalene-based compounds, one example of which is N,N′-di(n-1H,1H-perfluorooctyl) perylene-3,4,9,10-tetracarboxylic acid diimide. Substituents attached to the imide nitrogens in the diimide structure comprise alkyl chains, electron deficient alkyl groups, and electron deficient benzyl groups, the chains preferably having a length of four to eighteen atoms. Devices based on materials having a perylene framework used as the organic semiconductor have led to low mobilities, for example 10−5 cm2/Vs for perylene tetracarboxylic dianhydride (PTCDA) and 1.5×10−5 cm2/Vs for N,N′-diphenyl perylene tetracarboxylic acid diimide (PTCDI-Ph). See Horowitz et al. in “Evidence for n-Type Conduction in a Perylene Tetracarboxylic Diimide Derivative” Adv. Mater. 1996, 8, 242 and Ostrick, et al. J. Appl. Phys. 1997, 81, 6804. See also US Patent publication 2006/0134823 A1 and 2006/0131564 A1 for perylene-based semiconductor materials in which the substituents on the imide nitrogens are aryl or phenylalkyl groups.
US Patent Publ. No. 2005/0176970 A1 to Marks et al. discloses improved n-channel semiconductor films made of mono and diimide perylene and naphthalene compounds, nitrogen and core substituted with electron withdrawing groups. Substituents attached to the imide nitrogens in the diimide structure can be selected from alkyl, cycloalkyl, substituted cycloalkyl, aryl and substituted aryl moieties. However, Marks et al. do not see any comparative advantage of using cycloalkyl groups on the imide nitrogens. Accordingly, mobilities obtained from perylene diimides containing of N-octyl and N-cyclohexyl are virtually indistinguishable (page 10, Column 1, Example 10). Furthermore, the highest mobilities reported in US Patent Pub. No. 2005/0176970 A1 to Marks et al. were 0.2 cm2/Vs. Marks et al. show no experimental data with respect to naphthalene compounds, but require that their core be dicyano disubstituted.
As is clear from aforementioned description development of new semiconducting materials, both p-type and n-type, continues to be an enormous topic of interest. So far, amongst a variety of p-type materials, the highest charge carrier mobility (ca. 1.0 cm2/V-s) in thin film transistors has been observed with pentacene. However, the poor stability and reproducibility of pentacene-based OTFTs limit pentacene's commercial potential. Recently, Anthony et al. have reported a series of solution processable pentacene and anthradithioene derivatives with silylethynyl-substituted structures (see Payne et al J. Am. Chem. Soc., 2005, 127, 4986). The stability and the charge mobility have been improved relative to that of the parent molecules, owing to the enhanced π-stacking crystal packing. This is one of several promising molecular design approaches currently being explored. A recent molecular modeling study (see Deng, et al. J. Phys. Chem. B 2004, 108, 8614) of the relationship between the charge mobility and crystal packing of pentacene indicates that higher charge mobility in pentacene molecule is possible with densely packed crystal structure, but to create such an ideal packing structure with pentacene is impossible.
Control of molecular packing on charge transport in organic semiconducting materials continues to be a factor under consideration in developing new materials. So far, the effects of bulk crystal structure and film deposition temperature remain probably the most effective methods for controlling thin film morphologies, and hence performance, of organic thin film transistors
There remains a need in the art for improved performance of organic semiconductor materials in organic thin film transistors and improved technology for their manufacture and use.
It is well understood in the art (e.g., Stereochemistry of Organic Compounds, E. L. Eliel, Chapter 8 (1962) McGraw-Hill Co.) that minimally constrained cyclohexane structures adopt a chair-like conformation as displayed herein below. In this chair conformation, ring hydrogens or substituents are disposed in either axial or equatorial orientations. The ring on the left in the figure shows the A groups in the axial orientations, nearly perpendicular to the general plane of the cyclohexyl ring, while the B-groups are displayed in equatorial orientations, more nearly co-planar with the general plane of the ring. In the case depicted, the rings can interconvert via a well-understood process, with the two forms establishing an equilibrium mixture represented as follows:

This mixture of the two chair forms can favor one conformation over the other based on the chemical nature of the substituents. Hypothetically, when A and B are the same, the mixture will be composed of 50% of each conformational component. When A and B are sufficiently different, however, the mixture may be viewed as completely one conformer. For example, in the case where a cyclohexane ring bears only one substituent, in virtually all known instances that substituent favors an equatorial orientation. In the general case of cyclohexane rings displaying two or more substituents, various conformational mixtures may result, depending on the chemical nature of those substituents. In the more specific case of 1,4-disubstituted cyclohexanes, the substituents may be stereochemically disposed either on the same side of the ring, the cis configuration, or on opposite sides of the cyclohexane ring, the trans configuration. In this latter case (the trans configuration) due to the above considerations, the two trans-substituents can adopt either an axial-axial or an equatorial-equatorial conformation, with this latter conformation predominating in virtually all known instances of such a case. In the former case (the cis configuration), due to the above considerations, the two cis-substituents can only adopt an axial-equatorial configuration, however, in which one of differing substituents can be either in the axial or equatorial position. More particularly, a substituent that is on the C-4 position relative to a larger ring system such as in a naphthalene tetracarboxylic diimide ring system, the conformation in which the larger ring system is equatorial and the C-4 substituent is axial tends to predominate
The configuration as well as the conformations of substituted cyclohexane derivatives can be analyzed using a variety of spectroscopic techniques, e.g., see L. M. Jackman's Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry, 2nd Edition, Pergamon Press (1969) p. 238. In practice the preparation of very pure cis or trans-1,4-substituted cyclohexanes can be problematic, and often mixtures, to some extent, of cis and trans substituted compounds are prepared. In the present case, mixtures that are more than 70 mole percent, preferably more than 80 mole percent, and more preferably more than 90 mole-percent trans will be considered essentially pure trans. Similarly, for the cis configuration, the term “essentially pure” will refer to the same mole percents, as determined by Nuclear Magnetic Resonance Spectroscopy (NMR).